1. Field of the Invention
The field of the invention is a data modifying apparatus for use with a dual wavelength spectrophotometer.
2. Background of the Invention
Heretofore dual wavelength spectrophotometers have been utilized to pass two monochromatic light beams through a sample cuvette and a reference cuvette respectively, while changing the wavelength over a given wavelength span and recording a difference spectrum, i.e., the difference in absorbance by the sample and reference at the different wavelengths. An example of such a spectrophotometer is the spectrophotometer sold by Baxter Travenol Laboratories, Inc., of Deerfield, Ill., under the trademark DW-2.
In the spectrophotometer, light from a light source is split and directed along two optical paths while the wavelength of the light is changing. In the first path is located a reference station with a reference cell and in the second path is located a sample station with a sample cell. The light that passes through the sample cell is received by a photosensitive device as is the light which passes through the reference cell. The log of the signals generated by the light passing through the sample cell is subtracted from the log of the signals generated by the light passing through the reference cell and the log difference signal is plotted on a recorder.
When the sample cell and reference cell are removed from the spectrophotometer and only air is at the respective sample station and reference station, the difference between the signals generated by light that traverses both of the optical paths should be zero. However, due to different losses of light at different wavelengths in each of the optical paths, the difference between the two sets of signals at different wavelengths varies. When the log of this difference is plotted it forms a wavy baseline rather than a straight baseline. To correct for these differences in light losses in the respective optical paths at different wavelengths it has been proposed to utilize a plurality of potentiometers which are adjusted to render the wavy baseline as close as possible to a zero baseline so that as correct data as possible is obtained when light is passed through a sample and a reference and a difference spectrum is recorded.
The adjustment of such potentiometers can be time consuming and is never completely accurate, since a limited number of potentiometers cannot accurately compensate for every point of interest.
Also various methods and apparatus have been proposed, using tape recorders and associated electronic circuitry or mathematical function performing electrical circuitry for providing baseline or other corrections and for taking and displaying the derivative of a spectrum. Examples of such method and apparatus are disclosed in the following U.S. Patents.
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 3,561,872 Grabowski et al. 3,646,331 Lord 3,676,005 Chance 3,972,617 Shibata et al. ______________________________________
As will be described in greater detail hereinafter, the data modifying apparatus of the present invention provides electrical control, memory and processing circuitry: for receiving and storing baseline correction signals, for receiving and storing a difference spectrum; for subtracting the baseline correction signals from the difference spectrum; for causing a plot to be made of the corrected difference spectrum on a recorder or oscilloscope; for causing a cursor trace along the plot; for taking the derivative of the corrected difference spectrum; for subtracting one spectrum from another spectrum; and for adding two different spectra.